So-called conventional braking systems are known in practice. As a general rule, these comprise a brake pedal or a brake pedal/servo unit. Furthermore, the known conventional braking systems comprise a central hydraulic unit including an integrated electronic controller (for short: EBS), whereby the hydraulic unit has a pump in order to build up an active brake pressure independently of the driver in certain cases. Finally, braking systems of this sort have a hydraulically actuated wheel brake, which preferably comprises a brake disk and brake pads, associated with each wheel. Due to the application of the brake pressure produced by the pump, the brake pads surmount a clearance gap and come into contact with the brake disk; the brake “bites” and a braking action can be initiated until the brake is released again.
The wheel brakes and also the brake pedal or the brake pedal/servo unit are hydraulically connected thereby to the electronic hydraulic unit by appropriate hydraulic lines. Wheel revolution sensors detect the rotational speed of the wheels. Steering angle sensors detect the angle to which the steering wheel has been turned. A so-called “sensor cluster” that is frequently arranged in the middle portion of the vehicle floor detects different items of data that are specific to the vehicle, such as the yaw rate or transverse accelerations for example. Hereby, the electronic components are connected together by signal and/or power networks.
Further developments based on conventional braking systems of this sort are also known. In the case of so-called “electrohydraulic brakes” (for short: EHB), the hydraulic connection between the brake pedal and the central hydraulic unit developing the brake pressure is suspended in the normal operational case. Instead, the actuation of the brake pedal is detected electronically, the thus digitized requirement of the driver to decelerate is conveyed over a data line to the central unit of the hydraulic unit and, in dependence on the brake pedal signals, the hydraulic unit is addressed and activated accordingly. Hereby, the hydraulic unit in modern electrohydraulic braking systems of this sort has an additional hydraulic pressure accumulator. In consequence, a brake pressure requested by the driver can be built up abruptly since this pressure is already available in the pressure accumulator and there is no need to wait for the starting of the pump. The brake response times can be pushed down to quasi zero in such EHB systems. The braking distance can be shortened due to the faster build up of the brake pressure. In addition, greater comfort and a more pleasant feeling through the brake pedal can be established since this is no longer subjected to hydraulic feedback, this also being encapsulated in the expression “brake by wire” that is frequently used in EHB systems.
Moreover, electromechanical braking systems (for short: EMB) are also known. The electromechanical brake is the so-called pure form of the “brake by wire” technique. One can then dispense completely with brake fluid and hydraulic hoses. High powered electric motors produce the braking forces directly at the wheels. These are controlled by an electronic control unit. The actuating process is effected by means of an electronic brake pedal incorporating a pedal-feeling simulator and sensors for detecting the driver's requirements
However, electromechanical braking systems impose heavy demands on the electrical architecture of the vehicle, and especially on the availability of the on board electrical system and the electrical power supply.
Accordingly, there is a desire to be able to pass on the improvements and additional functionalities known from “brake by wire” systems to conventional braking systems, preferably in combination with the now standardly employed electronic stability programs (for short: ESP), and preferably in an economical manner by means of additional software and sensors should these be necessary. Preferably hereby, the additional functions known for improving active driving safety and comfort known from the EHB system should also be capable of being taken over by the ESP system. However, disadvantages are associated therewith due to the different constructional systems used for conventional and “brake by wire” brakes.
One of these known functions is the so-called “prefill” function. By a “prefill” function, there is meant the prefilling of the wheel brakes at a low pressure before the driver actively operates the brake pedal. This is intended to overcome the clearance gap before a braking action so that the brake pad is already resting on the brake disk prior to the operation of the brake pedal thereby losing less time for the stroke of the brake and thus for the development of an effective braking force during the braking action.
In the case of an electrohydraulic braking system, this “prefill” function is used successfully in series. Hereby, in the event of a rapid or abrupt release of the accelerator pedal, the wheel brake is pre-filled at low pressure for a certain standby period and hence the requisite residual time for applying the pads is pushed down towards zero since an emergency braking action is expected. The standby period for the wheel brake which is prepared for an emergency braking action in this way thereby extends in essence over the time from the abrupt releasing of the accelerator pedal to when the brake pedal is stepped on forcefully. Here, an EHB system takes advantage of the fact that the necessary pressure can be made available by the pressure accumulator and does not need to be produced by a pump. This circumstance is contrary to a simple transfer of the “prefill” function to conventional braking systems since pressure accumulators of this sort are not available therein. This means however that the pump must be activated immediately with the release of the accelerator pedal.
Since an electrohydraulic braking system is in fact a “brake by wire” system, the “prefill” function has no effect upon the feeling through the pedal. Rather however, this is simulated artificially and is freely selectable by taking the “back up” function into consideration. The resultant increase in fuel consumption due to the application of the brake pads to the brake disk does not have an appreciable negative effect due to the normally short length of the time period.
Due to the purely hydraulic connection between the brake actuating means and the wheel brake in a conventional braking system, the “prefill” function does have an effect upon the feeling through the pedal in a conventional braking system. In order to improve the feeling through the pedal when using this function, the brake pressure must be applied to an unlimited extent upon the release of the accelerator pedal in a conventional braking system, this thus leading to disadvantages in terms of fuel consumption and brake wear.